As emphasized in PAS-10-098, youth living with HIV (YLH) are a vulnerable subpopulation. The majority of YLH are of minority status, are likely to use illicit substances, and often struggle with adherence to HIV medications. The purpose of the proposed study is to test SMS text messaging technology to improve medication adherence among YLH. The proposed study is a randomized controlled trial of the effect of text message reminders on ART adherence among non-adherent YLH, ages 16-29. Daily text message reminders will be sent to patients randomized to the intervention group according to their medication schedule, for 6 months. A small feasibility study (N=25), including a high percentage of youth in need of alcohol and/or drug treatment (i.e., 60%), provided preliminary evidence of the potential effectiveness of this approach. We will enroll 152 non-adherent YLH, ages 16-29. Half of the sample (N=76), randomized to the intervention, will receive daily SMS text message medication reminders and half will be randomized to the control condition (N=76) and receive standard of care (SOC) only. For the controlled trial, adherence levels and viral load will be collected at baseline, 3-month and 6-month follow-up. In addition, at the end of the initial 6-month enrollment period, participants in the control condition will cross-over to the SMS intervention and participants in the intervention condition will cease to receive the SMS intervention. Adherence and viral load data will be collected from each group at 9- and 12-month follow-up points. The advantages of these additional features are that they allow us to offer the intervention to all participants, to evaluate the intervention effect in the control group (to confirm the intervention effect) and to assess sustained intervention effects in the intervention group (post-intervention). We hypothesize that youth in the intervention condition will demonstrate a clinically meaningful increase in adherence at 3 and 6 months post-baseline, from approximately 70% to 90% adherence to ART. Our specific aims are: 1) to determine the efficacy of an SMS text messaging intervention on our primary outcome: adherence to ART among poorly adherent YLH, ages 16-29 (a group shown to be at high risk for substance use) over a 6-month intervention period; 2) to explore the efficacy of an SMS text messaging intervention on a secondary, exploratory outcome: viral load and CD4 counts, over a 6-month intervention period; 3) to assess the feasibility, acceptability, and satisfaction of this intervention and clinical trial approach by both objective and subjective measures. Our primary hypotheses are: 1) youth randomized to the SMS text messaging intervention will show greater improvements in levels of adherence to ART and secondary outcomes during the intervention period at 3 and 6 months post-baseline, in comparison to those receiving SOC only. We hypothesize a 10-point difference in adherence level between groups at 3 and 6 months for the intervention to be clinically meaningful; 2) youth randomized to SMS text messaging intervention will maintain improvements (i.e., > 90% adherence) at 9-month and 12-month follow-ups.